1. Field of the Invention
This invention relates to a process for the manufacture of plastic moldings with a decorative coating, by injection molding in an injection molding machine, which injection molding machine has an injection screw which introduces the plastic into the cavity located in a mold clamping device. This invention also relates to an injection molding machine with an injection screw, which is connected by closable hot runners to the cavity of a clamping device consisting of a stationary mold part and a movable mold part, including the drive mechanisms, and with measurement and control components for the control of the process.
2. Background Information
The application of a decorative coating in the form of textiles, leather, films etc. to the surface of plastic parts is generally done by lamination. As an alternative to this relatively labor-intensive process, the bond between the materials can also be achieved in a single operation by rear injection. That is, the plastic can be injected directly onto the decorative coating, which decorative coating is placed within the mold.
The article from "Kunststoffe 80" (1990) 9, Page 997 ff. "Hinterspritzen von Dekormaterialien durch Niederdruckspritzgieaen" [Rear injection of decorative materials by low pressure injection molding] discloses a method of directly bonding the plastic and decorative materials to one another by injection molding, whereby the decorative materials, several millimeters thick, are rear injected with a thermoplastic material. That is, the decorative material is placed in the mold, so that the decorative material is bonded directly to the plastic during the injection process. This publication also describes pre-testing methods for the rear injection. This publication does not indicate any measures to minimize the pressure to prevent sink marks on ribs and changes in the wall thickness. That is, the decorative coating can be damaged and deformed during the injection process if pressures are too high.
At the Technical Conference on Decorative Moldings held at the South German Plastics Center on Nov. 15 and 16, 1990, reference was made to the use of injection compression devices or swelling molds. This publication also describes the use of a controlled hot runner with long flow distances, and thus the use of cascade technology. A precision regulation system for the data relevant to the injection with graphic display capabilities is also mentioned, and reference is made to the injection pressure. This publication contains no indication that the measurement factor to be used for the cascade control might be the distance travelled by the injection molding screw of the injection molding machine.
German Published Patent Application No. 39 27 995 relates to a process for the control of the holding pressure during the injection molding of thermoplastic plastics. The application discloses nozzle runners which can be closed by shutters, which nozzle runners inject molding compound into the cavity of the mold, and pressure signals which can be used to record the distance measurement signal from the injection screw (Column 4, Lines 9-17). This process, in which one or more desired molding parameters are set as a function of several process parameters recorded during at least one learning cycle to establish the mold-plastic pair, is complex and expensive.